Parabolic reflector antennas are widely used as satellite television antennas owing to a number of factors, including:
low cost;
wide band of operating frequencies;
simple work with waves of different polarizations;
relatively high aperture efficiency (AE) (usually 60-65 percent).
A parabolic antenna comprises a main reflector of which the surface is the result of parabolic movement along a trajectory in 3D space. The most common type of such reflector is the result of parabolic generatrix rotation around the axis passing through the parabola apex and focus. The parabolic antenna feed is located in the parabola focus. Thus a directional pattern with one main maximum (beam) is formed in the direction of the parabola axis. The shortcomings of parabolic antennas of this type are the characteristics of its single-beam and big size.
The big size of antennas creates the following shortcomings:
When those antennas are installed outdoor, they distort the architectural image of buildings. In particular, some countries of the European Union adopted legislation limiting installation of parabolic antennas on building walls and roofs.
Parabolic antennas can be hardly, if not at all, used on mobile carriers, especially when signal reception should be ensured in moving cars, trains, ships, etc.
When fixed near balconies or windows, antennas lead to excessive light blockage.
Under those circumstances, there is a need to develop plane and compact multibeam antennas for receiving a satellite television signal, having significantly smaller dimensions and ensuring simultaneous reception of signals from several satellites.
Dual-reflector antennas are more compact than parabolic reflector antennas. Unlike single-reflector parabolic antennas, which comprise one main reflector transforming a near-spherical feed wave front propagating from the feed to a plane wave front propagating from a big reflector, dual-reflector antennas comprise two reflectors—a big (main) reflector and a small (auxiliary or sub-reflector) reflector. Dual-reflector antennas solve the same task—they transform a near-spherical wave front of the feed into a plane wave front of the main reflector. However, the availability of an additional degree of freedom, namely a sub-reflector, makes this wave transformation more adaptable and enables the resolution of more complex problems in the field of achieving better electrical and dimensional characteristics of an antenna. There are different types of dual-reflector antennas, e.g., Cassegrain types of antennas, Gregory types of antennas, etc. They differ by their ray tracing distribution going from the feed to the sub-reflector and then to the main reflector. In Cassegrain type antennas beams going from the central part of the feed wave front come to the central part of the main reflector, and beams going from a lateral part of the feed wave front come to a lateral part of the main reflector.
An ADE (axially displaced ellipse) antenna is known (GB Patent #973583, publ. 1964). This antenna comprises a main reflector, a sub-reflector and a feed. The main reflector and the sub-reflector are made as bodies of revolution with a common revolution axis. The revolution axis is the axis 0z. The generatrix of the main reflector is a parabola. It is important that the parabola focus is not on the axis of revolution. The generatrix of the sub-reflector may have an arbitrary shape. In one case, a sub-reflector with an elliptic generatrix may be provided as in GB Patent #973583. This technical solution uses the following arrangement of the ellipse and parabola focuses: one ellipse focus coincides with the parabola focus, and the other ellipse focus is located at the axis of revolution.
Apart from antenna systems using a parabola and an ellipse as generatrices (as, for example, in the foregoing ADE system), there also exist other antenna systems with beam path inversion (inversed ray tracing). The feed field for reflector antennas may be represented as a totality of beams radiating from a point (feed phase center) in a limited space sector. In systems with inversed ray tracing, the feed field propagating from the central part of the radiation sector is reflected by a sub-reflector to a peripheral part of the main reflector, and the feed field propagating from a peripheral part of the radiation sector is reflected by a sub-reflector to the central part of the main reflector. Herewith, the main property of reflector antennas is maintained: the feed field is transformed into a locally plane wave propagating from the main reflector aperture. The constructive synthesis methods for generatrices of a system with beam path inversion are well known in the art. Such synthesis may be fulfilled by setting a feed directional pattern, space coordinates of the feed phase center and starting points of the reflector surfaces (e.g., for the central beam). Further, by moving along the angle coordinate from the central direction, one may obtain a surface shape for a system with beam path inversion from the condition of beam paths length equality. Generatrix pairs in such systems may be used instead of the “parabola-ellipse” pair for systems similar to ADE systems.
An antenna is known, which comprises a main reflector with a parabolic generatrix and a sub-reflector with an elliptic generatrix, forming a circle and a peak facing the main reflector and located between the circle and the main reflector (RF Patent #2296397, publ. 2006). The feed is located at the longitudinal axis of symmetry in the main reflector base between the main reflector parabolic surface and sub-reflector. This is a traditional ADE antenna optimized for obtaining maximum gain factor at minimum antenna thickness. Minimum antenna thickness (ratios H/D of 0.2-0.25 were obtained, where H is antenna thickness, D is main reflector diameter) is provided by special ratios between reflector parameters of the main reflector determined in the said RF Patent.
One limitation of the above single reflector and dual-reflector antennas with one feed, which are designed for satellite television systems, is their single main beam characteristic. An antenna has one input made as, e.g., a waveguide of a circular or other shape, and it has a directional pattern with a narrow main lobe oriented along the antenna axis of revolution. Such an antenna receives (transmits) signals mainly in a sector of angles corresponding to the main lobe of directional pattern. At the same time, many applications require multiple simultaneous reception or transmission of signals from multiple directions without turning the antenna or changing its configuration. This situation occurs, e.g., in receiving a satellite television signal. This situation is typical when several satellites work simultaneously at different azimuth angles (elevation bearings for all satellites at geostationary orbits are equal). Therefore, an antenna capable of receiving signals from several satellites without changes in configuration or mechanical rotation expands the capabilities of a satellite television reception system and, in particular, increases a number or information capacity for a number of channels receivable per one antenna.
Multibeam parabolic antennas have additional capabilities in comparison with single-beam parabolic antennas. Multibeam antennas use several feeds which are, as a rule, located near the focus. In this case, several directional patterns (beams) are formed in different directions, wherein each of them relates to its feed. As advantages, such antennas have multibeam characteristics, i.e., the capability to transmit and receive signals from various directions from/to one main reflector simultaneously as well as forming a complex-shape directional pattern consisting of a plurality of main lobes. The latter property, in particular, is widely used in satellite-based transmitting antennas.
A multibeam antenna is known (RF Patent #2173496, publ. 2001), which, in particular, was used in satellite television systems. This antenna is built according to a dual-reflector layout. In this antenna, the generatrix of the main reflector is a parabola, the generatrix of the sub-reflector is an ellipse, and the reflector surfaces are formed as a result of the generatrices spatial revolutions around axes orthogonal to the direction of the main lobe. Radiation sources are located at a spatial focal curve.
A disadvantage of this antenna is its large dimensions. It is connected with the fact that it has a rather high efficiency only in cases where its reflectors are long-focus. The long-focus characteristic of an optical system is usually determined by a ratio between the focal length of a parabolic main reflector F to its diameter D or to another typical dimension.
For the purpose of improving the antenna characteristics, such as directivity gain (DG), level of side lobes, etc., an antenna system may be shaped according to the offset layout, and systems “feed—sub-reflector”, as in the Cassegrain dual-reflector system, may be used as feeds of the main reflector. The closest technical solution to the inventive antenna is an offset system for satellite signals transmitting (JP4068803), wherein the main reflector representing a cut from a paraboloid of revolution is radiated with a plurality of horns with a corresponding formation of a plurality of partial directional patterns (DP) in a variety of directions. For the purpose of improving the properties of partial directional patterns, each horn is provided with one or two additional sub-reflectors.
The disadvantages of this antenna include its large dimensions due to a great ratio between its focal length and diameter F/D, a small angular distance between main lobes of partial directional patterns (DPs), a relatively low aperture efficiency (AE), and mutual blockage of “feed—sub-reflector” systems.